1. Field of the Invention
The present invention relates to a pneumatic booster used in a braking system of an automobile, and more particularly, it relates to a pneumatic booster of the type wherein the atmosphere and compressed air can be selectively supplied into a variable pressure chamber of the booster.
2. Related Background Art
A typical conventional pneumatic booster comprises a housing, a diaphragm for dividing the housing into a constant pressure chamber and a variable pressure chamber, and a valve body connected to the diaphragm. The valve body is provided with a bore within which a plunger is slidably mounted for movement in an axial direction. The plunger is provided with two passages so that, when the plunger is slidably shifted relative to the valve body, the variable pressure chamber can selectively be communicated with the atmosphere or a compressed air source through the respective passages via a valve mechanism.
Since the above-mentioned two passages extend in an axial directions within the plunger, respectively, a diameter of the plunger is naturally increased. Thus, in the conventional pneumatic booster, there arose a problem that the entire booster becomes bulky.